1. Field of the Invention
The present invention relates to a method of fabricating a silicon substrate for use in manufacturing magnetic recording media, and a magnetic recording medium including a magnetic recording layer on the silicon substrate obtained by the fabrication method.
2. Description of the Related Art
In the technical field of information recording, a hard disk device as means for magnetically reading/writing such information as letters, images, and music, is now indispensable as a primary external recording device or built-in type recording means for use with or in electronic devices including a personal computer. Such a hard disk device incorporates therein a hard disk as a magnetic recording medium. Conventional hard disks have employed a so-called “in-plane magnetic recording system (longitudinal magnetic recording system)” which is configured to write magnetic information on a disk surface longitudinally.
FIG. 1A is a schematic sectional view illustrating a typical stacked layer structure for a hard disk of the longitudinal magnetic recording system. This structure includes a Cr-based underlayer 2 formed by sputtering, a magnetic recording layer 3, and a carbon layer 4 as a protective layer, which are sequentially stacked on a non-magnetic substrate 1, and a liquid lubricating layer 5 formed by applying a liquid lubricant to the surface of the carbon layer 4 (see Japanese Patent Laid-Open No. 5-143972 (Patent Document 1) for example). The magnetic recording layer 3 comprises a uniaxial magnetocrystalline anisotropic Co alloy, such as CoCr, CoCrTa, or CoCrPt. Crystal grains of the Co alloy are magnetized in a longitudinal direction of a disk surface to record information. The arrows in the magnetic recording layer 3 shown indicate directions of magnetization.
With such a longitudinal magnetic recording system, however, when individual recording bits are reduced in size to increase the recording density, the north pole and south pole of a recording bit repel the north pole and south pole, respectively, of an adjacent recording bit, to make the boundary region magnetically unclear. For this reason, the thickness of the magnetic recording layer needs to be decreased to reduce the crystal grain size for the purpose of realizing recording density growth. It is pointed out that as crystal grains are made more minute (i.e., reduced in volume) and recording bits made smaller in size, a phenomenon called “heat fluctuation” occurs to disorder directions of magnetization of crystal grains by thermal energy, thereby to cause a loss of recorded data. Thus, the recording density growth has been considered to be limited. The effect of the heat fluctuation becomes serious when the KuV/kBT ratio is too low. Here, Ku represents magnetocrystalline anisotropic energy of a magnetic recording layer, V represents the volume of a recording bit, kB represents a Boltzmann constant, and T represents an absolute temperature (K).
In view of such a problem, a “perpendicular magnetic recording system” is now studied. With this recording system, the magnetic recording layer is magnetized perpendicularly to the disk surface, so that north poles and south poles are alternately arranged as bound one with the other in recording bits. Therefore, a north pole and a south pole in a magnetic domain are positioned adjacent to each other, to strengthen the mutual magnetization. As a result, the magnetized state (i.e., magnetic recording) is highly stabilized. When a magnetization direction is recorded perpendicularly, a demagnetizing field of a recording bit is lowered. For this reason, the perpendicular magnetic recording system does not need to make the recording layer very thin, as compared with the longitudinal magnetic recording system. Accordingly, if the recording layer is thickened to have a larger perpendicular dimension, the recording layer, as a whole, has an increased KuV/kBT ratio, thereby making it possible to reduce the effect of the “heat fluctuation”.
Since the perpendicular magnetic recording system is capable of lowering the demagnetizing field and ensuring a satisfactory KuV value as described above, the perpendicular magnetic recording system can lower the instability of magnetization due to the “heat fluctuation”, thereby making it possible to expand a margin of recording density substantially. Therefore, the perpendicular magnetic recording system is expected to realize ultrahigh density recording.
FIG. 1B is a schematic sectional view illustrating a basic layered structure for a hard disk as a “double-layered perpendicular magnetic recording medium” having a recording layer for perpendicular magnetic recording which is stacked on a soft magnetic backing layer. This structure includes a soft magnetic backing layer 12, a magnetic recording layer 13, a protective layer 14, and a lubricating layer 15, which are sequentially stacked on a non-magnetic substrate 11. Here, the soft magnetic backing layer 12 typically comprises permalloy, amorphous CoZrTa, or a like material. The magnetic recording layer 13 comprises a CoCrPt-based alloy, a CoPt-based alloy, a multi-layered film formed by alternately stacking several layers including a PtCo layer and ultrathin films of Pd and Co, or the like. The arrows in the magnetic recording layer 13 shown indicate directions of magnetization.
The hard disk of the perpendicular magnetic recording system includes the soft magnetic backing layer 12 underlying the magnetic recording layer 13, as shown in FIG. 1B. The soft magnetic backing layer 12, which has a magnetic property called “soft magnetic property”, has a thickness of about 100 to about 200 nm. The soft magnetic backing layer 12 is provided for enhancing the writing magnetic field and lowering the demagnetizing field of the magnetic recording film and functions as a path which allows a magnetic flux to pass therethrough from the magnetic recording layer 13 while allowing a magnetic flux for writing to pass therethrough from a recording head. That is, the soft magnetic backing layer 12 functions like an iron yoke provided in a permanent-magnet magnetic circuit. For this reason, the soft magnetic backing layer 12 has to be set thicker than the magnetic recording layer 13 for the purpose of avoiding magnetic saturation during writing.
Magnetic recording media are gradually switching from the longitudinal magnetic recording system as shown in FIG. 1A to the perpendicular magnetic recording system as shown in FIG. 1B as the recording density increases from a border which ranges from 100 to 150 Gbit/square inch because the longitudinal magnetic recording system has a limited recording density due to the heat fluctuation. Though the recording limit of the perpendicular magnetic recording system remains uncertain at present, the recording limit is estimated to ensure a value of not less than 500 Gbit/square inch. In another view, the perpendicular magnetic recording system can achieve a recording density as high as about 1,000 Gbit/square inch. Such a high recording density can provide for a recording capacity of 600 to 700 Gbites per 2.5-in. HDD platter.
Substrates generally used in magnetic recording media for HDDs include an Al alloy substrate used as a substrate having a diameter of 3.5 inches, and a glass substrate used as a substrate having a diameter of 2.5 inches. In mobile applications such as a notebook personal computer, in particular, HDDs are likely to frequently undergo impacts from outside. Therefore, a 2.5-in. HDD used in such a mobile application has a high possibility that its recording medium or substrate is damaged or data destroyed by “head-disk collision”. For this reason, use has been made of a glass substrate having a high hardness as a substrate for magnetic recording media.
As a mobile device is reduced in size, a substrate for use in a magnetic recording medium to be incorporated therein calls for a higher impact resistance. Substrates having small diameters of not more than 2 inches are mostly used in mobile applications and hence call for a higher impact resistance than 2.5-in. substrates. Also, the downsizing of such a mobile device inevitably calls for downsizing and thinning of parts to be used therein. The standard thickness of a substrate having a diameter of 2.5 inches is 0.635 mm, whereas that of a substrate having a diameter of, for example, 1 inch is 0.382 mm. Under such background circumstances, a demand exists for a substrate which has a high Young's modulus, ensures a sufficient strength even when made thin, and offers good compatibility with the magnetic recording medium fabrication process.
Though a glass substrate having a diameter of 1 inch and a thickness of 0.382 mm has been put to practical use by mainly using reinforced amorphous glass, further thinning is not easy. Further, since a glass substrate is an insulator, a problem arises that the substrate is likely to be charged up during a sputtering process for formation of a magnetic film. Though volume production is made practically possible by changing a holder holding a substrate to another one during sputtering, this problem is one of the factors making the use of a glass substrate difficult.
Study has been made of FePt having high magnetocrystalline anisotropy or the like as a material for a next-generation recording film. Such an FePt film needs to be heat-treated at a high temperature of about 600° C. so as to have a higher coercive force. Though studies have been made to lower the heat treatment temperature, a heat treatment at a temperature of not lower than 400° C. is still needed. Such a temperature exceeds the temperature at which currently used glass substrates can resist. Likewise, Al substrates cannot resist such a high temperature treatment.
Also, study has been made of discrete track media (DTM) and bit patterned media (BPM) aiming at improving the recording density by microfabrication of magnetic recording media. Such microfabrication includes techniques of the semiconductor field, such as an etching technique. Under the actual circumstances, it is difficult to subject the surface of a currently available glass substrate or Al substrate directly to such microfabrication.
Besides such glass substrate and Al substrate, alternative substrates have been proposed which include a sapphire glass substrate, an SiC substrate, an engineering plastic substrate, and a carbon substrate. However, the realities are such that any one of such substrates is inadequate for use as an alternative substrate for next-generation recording media in view of its strength, processability, cost, surface smoothness, affinity for film formation, compatibility with microfabrication, heat resistance, and like properties.
Under such background circumstances, the inventors of the present invention have already proposed use of a single crystal silicon (Si) substrate as an HDD recording film substrate (see Japanese Patent Laid-Open No. 2005-108407 (Patent Document 2) for example).
Such a single crystal Si substrate, which is widely used as a substrate for LSI fabrication, is excellent in surface smoothness, environmental stability, reliability, and the like and has a higher rigidity than glass substrates. For this reason, the single crystal Si substrate is suitable for an HDD substrate. In addition, unlike glass substrates having insulating properties, the single crystal Si substrate is semiconductive and has a certain electric conductivity because the single crystal Si substrate usually contains a p- or n-type dopant. Thus, the single crystal Si substrate can lessen the charge-up effect which occurs during film formation by sputtering to a certain extent and allows a metal film to be formed thereon by direct sputtering or bias sputtering. Further, since the single crystal Si substrate has good thermal conductivity and is resistant to high temperatures, the Si crystal substrate can easily undergo heating at 400° C. or higher and hence has very good compatibility with a sputtering process for formation of FePt film or the like which calls for heating at elevated temperatures.
What is more, the Si substrate has the advantage that its crystal purity is very high and its substrate surface obtained after processing is stable with a negligible change with time. Further, the silicon substrate, which is highly compatible with the semiconductor fabrication process, is applicable to next-generation recording media.
However, Si single crystals of the “semiconductor grade” for fabrication of such devices as LSIs are generally expensive. Further, the prices of single crystal silicon and polycrystalline silicon of the “solar grade” are soaring with increasing demand due to solar cells widespread in recent years. When consideration is given to use of a single crystal Si substrate as a substrate for magnetic recording media, a serious problem arises that the single crystal Si substrate is significantly inferior to glass substrates or Al substrates in terms of raw material cost.
The single crystal Si substrate has the property of cleaving in a specific crystal orientation (110). For this reason, when the single crystal Si substrate used in a mobile device or the like undergoes an external impact, the substrate might cleave. In this respect, the inventors of the present invention have confirmed that no practical problem will arise if end face polishing is improved. However, some probability of fracture remains.